1. Field of the Invention
The present invention relates to a battery that includes: a package can containing a battery element having a cathode and an anode; and an external connection terminal having the polarity different from that of the package can.
2. Description of the Related Art
In recent years, many portable electronic devices such as combination cameras (videotape recorder), mobile phones, and notebook personal computers have been introduced, and their size and weight have been reduced. For a battery used as a portable power source for these portable electronic devices, in particular a secondary battery as a key device, research and development has been actively promoted to improve the energy density. Specially, a nonaqueous electrolyte secondary battery (for example, lithium ion secondary battery) can provide a higher energy density compared to a lead battery and a nickel cadmium battery as an existing aqueous electrolytic solution secondary battery. Thus, studies of improving such a nonaqueous electrolyte secondary battery have been made in various fields.
As an anode active material used for the lithium ion secondary battery, a carbon material such as non-graphitizable carbon and graphite that shows the relatively high capacity and has the favorable cycle characteristics has been widely used. Meanwhile, as an anode with the higher capacity than that of the carbon material, researches on an alloy material have been promoted. In such an alloy material, the fact that a certain type of metal is electrochemically alloyed with lithium, and the resultant alloy is reversibly generated and decomposed is applied.
In the battery with the higher capacity as above, securing the safety is important more than before. For example, when some external force is excessively applied to the package can of the battery, that is, applied to the degree that the package can is deformed, resulting in short circuit between the cathode and the anode inside the battery element, an active material may generate a great deal of heat. Thus, in the past, the following mechanisms have been proposed. In one of the mechanisms, when a package can is deformed, short circuit is generated at a portion different from a battery element, and in another mechanism thereof, when a package can is deformed, current collectors are short-circuited (for example, refer to International Publication No. WO99/059213, Japanese Unexamined Patent Application Publication Nos. 2001-338636, 9-259926 and 2006-49312, and Japanese Patent Publication No. 3178586). These mechanisms intend to block the battery reaction and thereby safely terminating the power generation function.